This invention relates to the extrusion of materials which are sensitive to dehomogenization and degradation, and more specifically explosive materials that are extruded in the form of longitudinally perforated strands which are cut into pellet-size lengths. Typical materials encountered in the art with which this invention is concerned comprise highly viscous substances with entrained particulate solids. The material is discharged from either a continuous or batch mixer as a highly viscous fluid with entrained particulate solids of explosive materials, such as nitramines, encased in a synthetic plastic binder. A typical binding plastic is cellulose acetate butyrate with a solvent, which solidifies with solvent vaporization on being extruded from the multiple pin dies which are employed.
Continuous mixer-extruders of the type disclosed in U.S. Pat. No. 4,744,669, can be utilized to mix and extrude such energetic material. The machine disclosed in that patent is a twin screw, co-rotating, self-wiping, fully intermeshing, continuous mixer with in-line extrusion capability. Typical energetic materials may be identified as single-base propellants (nitro-cellulose), double-base propellants (nitro-cellulose plus nitro-glycerin or other liquid explosive), and triple-base propellants (nitro-cellulose plus nitro-glycerin plus nitro-guanidine). An alternate method of producing gun propellants is the batch processing system wherein the product of sigma blade mixing is fed to a ram extruder which supplies the mixed material directly to the dies. Prior art extruding systems have not provided the homogeniety desired and have been characterized by a considerable pressure drop in the dies which facilitated the formation of secondary flow currents and dead spots.